This application is a continuation-in-part of application Ser. No. 433,352 filed Jan. 14, 1974, now abandoned.
This invention relates to a novel electro-optical display device which involves a great number of liquid crystal display elements arranged in a predetermined order in a display cell, and wherein said elements are activated selectively and sequentially by an electrically controlled circuit so as to make an analogical fashion display, wherein a number of display elements can be turned ON sequentially one by one and keep the ON states accumulatively.
Recently, liquid crystals have been developed and employed in transmissive, reflective, or absorptive type flat panel displays utilizable for a light shutter or other applications. So far the classes of liquid crystal materials have been identified to be of three types: the cholesteric, the nematic and the smectic. Of the three types of liquid crystal materials, nematic liquid crystals have properties suitable for use in the device of the present invention. This nematic liquid crystals are further subdivided into two classes, the one is Dynamic Scattering Mode (DSM), and the other is Field Effect Mode (FEM). The Field Effect Mode liquid crystals are further classified into two types, i.e., the one is the liquid crystal with positive dielectric anisotropy and the other is the liquid crystal with negative dielectric anisotropy. Especially, the former one is called as a Twist Nematic Liquid Crystal.
These nematic liquid crystals are excited to exhibit a peculiar electro-optical effect, when either a Direct Current (DC) voltage or an Alternating Current (AC) electric signal is applied across a display cell composed of a pair of electrode plates and a liquid crystal material filled in-between. This electro-optical effect of the liquid crystal can be utilized as an effective display means. In the DSM type liquid crystal, as an electric voltage is applied thereacross, an electric field is induced in the liquid crystal to cause a turbulent movement of its molecules so that the incident lights are reflected at the cell in different directions, which is referred to as Dynamic Scattering Effect.
Whereas in the FEM type liquid crystal, as an electric voltage is applied across the liquid crystal layer, movement in the orientation of its molecules is caused. By the aid of a pair of polarlizers, this change can be observed and its physical aspect is utilized as an effective display.
Electric signals utilized for exciting such liquid crystal materials may be either a DC voltage supplied by a power cell or AC voltage which can be obtained from commercially available power sources. However, an electric signal in the form of a pulse with a constant voltage is preferred in application for this purpose. Such an electric signal is usually a DC pulse. However, as its nature, liquid crystal keeps longer life when driven by an alternating electric signal. Therefore, it is more advantageous to drive the liquid crystal by equivalent of AC voltage composited with such DC pulses.
For convenience in the explanation to follow, here is defined respectively terms "ON" to mean the state wherein the liquid crystal is excited to cause a display activity, and "OFF" represents the state wherein it is in quiescent condition and no display activity is performed. In the present invention, each of the liquid crystal display elements is adapted to be turned ON one after another in the predetermined sequence, and remains in the ON state to represent an accumulative display. Therefore, a quick and direct readout in analogue fashion can be obtained by glancing the display face and recognizing the number of display elements thus excited and in the ON state. This means that in place of alphanumerical time display in now commercially available digital watches, a unique time display representing analogically time indication in analogous to the watch hands in conventional timepiece has been realized.
Liquid crystal material requires such a specific voltage value of its own as sufficiently strong enough to excite it in accordance with its property, which is called a threshold voltage. Further, even if a voltage greater than this threshold voltage is applied across the liquid crystal material, it is not until a certain duration of time (referred to as the response time of the liquid crystal) has passed that it is excited to become to the "ON" state. Herein, the response time of the liquid crystal means the rising time and the falling time thereof; the rising time represents a length of time when it takes for the liquid crystal to attain to be fully in the excited state by the application of the required voltage, and the falling time is a length of time when it takes for the liquid crystal to be back from the excited state to the quiescent state by the removal of such applied voltage.
Between these two parameters, there is a correlationship such that the higher the voltage applied, the shorter the rising time. Consequently, the excitation voltage of a certain liquid crystal material is to be determined by both the strength of the voltage applied and the duration of time during which the liquid crystal is subjected to such a voltage. Therefore, the excitation voltage to turn ON the liquid crystal display elements means that the voltage which is greater than the threshold voltage of the liquid crystal is applied and the application of such a voltage maintains for the duration longer than the response time of the liquid crystal at such an applied voltage level. Whereas, the excitation voltage for turning OFF the display element represents the case either the applied voltage is lower than the threshold voltage, or even if the applied voltage is higher than the threshold, the time duration of the application of such a voltage is shorter than the response time of the liquid crystal at this applied voltage. The drive pulse of liquid crystal employed in the present invention has the excitation voltage which satisfies the above-mentioned requirements. Therefore, here we define a pulse with such an effective value of the voltage as to sufficiently make the excitation of the liquid crystal to be "a turn-ON pulse" and a pulse without such an effective value of the voltage to be "a turn-OFF pulse".
In the display device which comprises a number of liquid crystal display elements, these elements are composed of a layer of liquid crystal material confined between a pair of electrodes. Said electrodes pairs are provided with generally one common electrode and a plurality of segment electrodes. Further, one terminal lead is brought out from said common electrode and each one terminal lead is brought out from the respective segment electrodes. These leads are connected respectively to the predetermined switching means corresponding to said electrodes. By the operation of said individual switching means, each display elements can be driven and turned ON or OFF. When one display element is turned ON, a predetermined voltage is applied to said display element so that an electric field may be produced between said pair of electrodes and the liquid crystal can be fully excited to represent an electro-optical display. When one display element is turned OFF, no voltage is applied to said display element whereby an electric field is not produced between said pair of electrodes and the liquid crystal maintains the quiescent state to represent no display activity.
In order to excite the liquid crystal and drive the display element for turning ON or OFF, there have been developed two main conventional driving techniques; one is the Static Drive Method and the other is the Dynamic Drive Method. In the case of the Static Drive Method, a predetermined voltage of either DC or AC signal is continuously applied to a pair of electrodes for a certain duration of time, and by producing potential difference between said electrode pairs at all times while said voltage is applied, the display element is able to maintain the excited state of the liquid crystal so as to turn ON the display element. In the case of the Dynamic Drive Method, it is only for a momentary duration of time that a predetermined voltage is applied across one of the display elements, and the display element can be turned ON only for said momentary duration of time. But such a voltage is applied repeatedly every predetermined time intervals so long as the display element is required to remain in the ON state. By the switching operation of which such application of the voltage is repeated cyclically, especially at very high speed, it produces the effect that the display element subjected to said voltage looks like being in the continuous ON state because flickering of the display activity cannot be observed by the human eyes.
Referring to such conventional display devices which are provided with the above-mentioned drive methods, if a great many of display elements are intended to be controlled sequentially in order to perform an effective display information, the switching operation will be much complicated. That is the reason that it is necessary to select sequentially the display elements to be displayed again and again everytime the display information is desired to be changed. Moreover, the great number of leads and switches are necessary because the same number of leads as the total number of a common electrode and a plurality of segment electrodes are required, and the same number of switches as the number of said segment electrodes are required.
For the purpose of reducing the number of such leads and switches, a matrix scanning method has been developed and utilized. In this matrix scanning method, all the display elements are provided at the intersections formed by a number of row electrodes and column electrodes. Therefore, the respective number of the leads and switches needed for this method is sufficient for the sum of the number of the rows and columns. With respect to the switching operation of this method, when row selector circuits select the required row, electrodes for energizing their conductive lines of electrodes, column selector circuits select the required columns electrodes, sequentially one after another in a predetermined order. Therefore, in this switching technique, it is only one display element or a few display elements positioned on one conductive line electrode that can be selected to be turned ON in a certain moment.
On the contrary, when an accumulative and sequential activation of the display elements is intended in the application of this matrix arrangement, a time sharing drive method equipped with a multiplex switching operation must be employed. However, in the case of which a great number of liquid crystal display elements are needed to be controlled, this time sharing drive method cannot be utilized in the practical application, because the time duration when each display element receives an applied voltage is extremely reduced, and the liquid crystal cannot respond to such a short applied voltage. This will result in little or no excitation of the liquid crystal, so that the display element cannot be fully turned ON.
By eliminating the above mentioned defects of the conventional display techniques, the present invention presents a novel and progressive display device wherein a great number of liquid crystal display elements can be activated to be turned ON sequentially and accumulatively one by one in a predetermined sequence, and the display elements being in the turned ON states exhibit an analogical fashion display. Accordingly, said conventional matrix scanning method and the time sharing drive method cannot be utilized for the present invention.